Within a series there can be variations. As an example, in series 1, there are three different layouts for the back of the card. At the bottom of the card one variation reads "Mount your collection..." while another reads "Albums available at your grocers..." while the final variation reads "Save your cards in a colorful album..." Of course Bill has to collect each variation.
Don't forget the albums. It's not just the cards that get alternate printings.

Some of the cards come as "Red Tops." This means on the back the top line is in red instead of the usual black top line. Series 7 only comes in "Red Top."

We know that blue backed (or cards with blue ink on the
description side) are US and that (for the most part) black backed cards are
Canadian.

Here is a comprehensive list of variations that I am aware of.
Don't forget that there are errors, too. Check them out on the errors
page.